This invention relates to a method of measuring the tritium activity of a drum of radioactive waste.
Tritium is a radionuclide present in radioactive waste. It is an unstable isotope of hydrogen. Its decay into 3helium is accompanied by the emission of a xcex2 particle with a negative charge which corresponds to an electron. The distances traveled by these xcex2 particles are much reduced. They do not exceed 6 xcexcm in water and 5.7 mm in air. This property excludes any possibility that the radiation from the tritium could be detected through the walls of the drum or within the solid or liquid waste.
In order to measure the amount of tritium in the radioactive waste, other methods of measurement must be used that exploit the main characteristics of tritium that enable its presence to be detected. Its atomic mass permits its separation by mass spectrometry and by chromatography, if it is in the gaseous phase. Thanks to the xcex2 radiation, it is possible to make use of the scintillation counting technique, most commonly in a liquid medium. The absorption of xcex2 radiation in the surrounding medium induces a release of heat. This flow of heat can be quantified by calorimetry.
In the majority of applications, it is these physical and chemical properties which are exploited in order to measure the tritium. The field of tritiated waste is not outside this general rule. Before being able to use the techniques mentioned above, it has often proved necessary to proceed via steps involving the preparation of tritiated samples. These preliminary phases, which are sometimes complicated, depend on the physical and chemical condition of the tritium and the matrix within which it is trapped. The level of activity being looked for, the chemical form of the tritium and the matrix therefore define the criteria for choosing the measurement techniques to be used.
Tritium is also present in organic solid waste, for example in polyethylene/vinyl acetate or PEVA or in polyvinyl acetate or PVC, arising from overshoes, gloves, inner gloves etc. The diversity of this waste reinforces the difficulty of characterizing it. Experience has shown a high degree of heterogeneity in the distribution of the activity. When all these types of products are mixed, the chance that a few grams of samples can be representative is very uncertain. Nevertheless, burning a sample whose weight ranges from 0.1 to 1 g under a current of oxygen enables one to estimate the activity of some samples at levels of a few Bq/g.
Measuring the dose of tritium in a gaseous or aqueous phase makes use of traditional techniques which have been well proven, the most common being scintillation and mass spectrometry. When the medium is homogeneous, simple sampling enables one to obtain a reliable quantitative result within an exhaustive range of concentrations.
However, the determination of the activity of relatively slightly contaminated tritiated waste remains particularly difficult. In this precise case, the use of destructive analytical techniques is not satisfactory because of the relative uncertainty of how representative the sample is. Up to now, no simple reliable method has existed to characterize technologically tritiated waste.
Rigorous and effective management of tritiated waste requires the measurement of the quantity of tritium contained in the parcel. For a specialist body to take responsibility for this waste, the tritium activity must be guaranteed to be less than a defined threshold, for example, 109 Bq (0.027 Ci) in a 200 liter drum. No reliable technique that is economically acceptable enables one to fulfil this requirement at the present time. Calorimetry on 200 liter drums can be carried out but this does not permit one to measure tritium activities less than 1.8xc3x971014 Bq (5000 Ci).
Unless it is in liquid or gaseous form, slightly contaminated tritiated waste is very difficult to characterize, especially organic waste, each sampling of which is fraught with great uncertainty as to whether it is representative. It is possible that the problem can be resolved by homogenizing the waste by grinding it. The lack of sensitivity of calorimetry does not allow it to be used. From the point of view of analytical controls, an important deficiency continues to exist. The solution is to arrive at an overall control of the drum that is non-destructive and does not generate waste.
In order to remedy this problem, according to this invention a technique is proposed that is non-destructive, reliable and economic and which enables one to quantify the activity of tritiated waste enclosed in bags, for example PVC bags, themselves placed within a drum with an unknown free volume and with unknown leakage.
The principle consists of measuring the quantity of 3helium arising from decay of the tritium, from a sample taken from the gaseous atmosphere surrounding the waste. This quantity of formed helium is proportional to the tritium activity present. By way of example, waste with an activity of 109 Bq (0.027 Ci) gives rise, in one year, to a release of 3helium leading to a concentration of 0.0055 ppm of this gas in a volume of 200 liters.
Tritium is xcex2xe2x88x92 radioactive. It decays giving 3helium, an electron and an anti-neutrino according to the reaction:
Txe2x86x923He+exe2x88x92+{overscore (xcexd)}
The half life of the tritium is 12.34 xc2x10.02 years. The number of atoms of 3helium generated is directly related to the number of atoms of tritium present through the equation:
N3He=NT.(1xe2x88x92exe2x88x92xcext)
where N3He=the quantity of 3helium formed during time t,
NT=the quantity of tritium initially present,
xcexc=the radioactive constant of tritium.
Hence the subject of the invention is a method of measuring the tritium activity of a drum of radioactive waste containing a quantity of radioactive waste and a free volume. The method consists of measuring the quantity of 3He produced by the decay of the tritium contained in said radioactive waste over a defined period of time and deducing from this the corresponding activity of the tritium contained in said radioactive waste. The quantity of 3He produced may advantageously be evaluated by a leak detector.
The tritiated waste enclosed in the bags, taken overall comprise a single source of 3helium which flows into the free volume of the drum. The partial pressure of the gas present is a function of the flow rate of 3helium from the tritium source (and hence the activity), the free volume, the tightness of the seal of the drum (a part of the gas created will escape from it) and the confinement time.
According to the invention, the measurement can be carried out in three steps: calibration and measurement of the concentration of 3helium, measurement of the free volume in the drum and of the tightness of the drum.
The method according to the invention may therefore comprise the following operations:
a) the taking of a sample of the gas contained in the drum and evaluation of the quantity of 3He contained in the sample using a leak detector,
b) measurement of the free volume of said drum,
c) measurement of the tightness of the drum to determine the leakage rate from it,
d) calculation of the flow rate of 3He using data obtained from operations a), b) and c),
e) determination of the tritium activity of the drum in relation to the flow rate of 3He calculated in operation d).
Preferably, during operation a), parasitic gases are removed from the sample before making the evaluation of the quantity of 3He. During this operation, said evaluation may comprise the comparison of the sample taken with a gas at the same pressure and having a known 3He concentration.
Operation b) can be carried out by injecting a known quantity of 4He into the drum and then measuring the partial pressure of 4He in the drum, and finally by determining the free volume of said drum from the known quantity of 4He and the measurement of the partial pressure of 4He. This measurement of the partial pressure of 4He can be obtained by taking a sample of the gas contained in the drum and evaluating the quantity of 4He contained in this sample using a leak detector.
Operation c) may be carried out by injecting a known quantity of 4He into the drum, then measuring for the first time the partial pressure of 4He in the drum and then, after a period of time defined in relation to said first time, measuring the partial pressure of 4He in the drum for a second time. The rate of leakage from the drum is then calculated from these values of the partial pressures of 4He and said defined period of time between these partial pressure measurements.
In the case where the apparent leakage of 3He from the drum is estimated as being equal to the quantity of 3He produced through the decay of the tritium contained in said radioactive waste, the measurement of said quantity of 3He produced may be simply obtained by placing the drum within an enclosure intended to collect the 3He leaking from the drum and by evaluating this quantity of 3He using the leak detector.
The invention will be better understood and other advantages and particular features will become apparent on reading the description that follows, given by way of a non-limitative example, accompanied by the appended FIGURE which illustrates a method of measuring the tritium activity of a drum of radioactive waste, in accordance with the present invention.